Since the second generation of computers in the 1950's, until the rise of surface-mount technology, through-hole soldering has been the predominant process for mounting integrated circuit (IC) and other electrical components on printed circuit boards. Even though surface-mount technology has largely replaced through-hole circuit board assembly, however, through-hole technology remains a significant part of electronic device manufacturing in various technology areas and for various components that are still manufactured in through-hole packaging. Through-hole technology refers to an electronic component mounting process, whereby electrical conductor leads of a component are inserted through holes drilled in the printed circuit board (PCB), and are soldered to conductor pads on the board either by manual assembly or automated insertion mount machines.
FIGS. 1A, 1B and 1C show various views of a typical through-hole PCB assembly/solder process. FIG. 1A illustrates a top view of a section of a printed circuit board (PCB) 101 with the components (an IC 110, a coil 112 and a capacitor 114) inserted into the PCB for a typical through-hole PCB component assembly/solder process. FIG. 1B illustrates a cross-sectional view of the PCB section of FIG. 1A (along the line X-X′), showing a cross-section along the through-holes of the PCB and the component pins inserted through the through-holes FIG. 1C illustrates the cross-sectional view of the PCB section of FIG. 1B, after completion of the typical through-hole PCB component solder process, along with an expanded view of a single solder joint and wiring layers of the PCB. With reference to FIG. 1B, in a typical through-hole process, the pins or electrical conductor leads of the component (e.g., the pins 116 of the IC 110, the pins 118 of the coil 112, the pins 120 of the capacitor 114) are inserted through plated holes 122 in the PCB, such that pin protrusions 124 from the component leads extend through the back side of the PCB 101. When soldered, for example, through a wave solder process, the component leads (including the pin protrusions through the back of the board) and the plated through-hole form a solder joint. During the normal process, the boards are first run through a surface-mount stencil printer that applies a solder paste over the top side of the board, forcing the solder paste into the plated through-holes of the PCB 101. After the solder paste is applied, the board is run through a component placement machine that places the surface mount (SMT) components, and then operators place the pin-in-hole (PIH) components by inserting the component leads into the respective PCB through-holes 122. The PCB 101 is then put through a solder reflow oven (or in other applications/processes, a solder wave bath), which heats the board and melts the solder paste to form a solder joint between each component lead and the metalized portion of the respective through-hole. This process leaves protruding component pins on the bottom of the printed circuit card assembly (PCBA) with a conical shaped solder joint attached to the annular ring of the metal through hole.
With reference to FIG. 1C, and the expanded view of a single solder joint as a result of the solder reflow process, ideally, the solder flows into the through-hole and fills a space between the component leads and the metalized surface of the through-hole 122. Additionally, the solder paste forms a protruding conical-shaped solder joint 128 around the pin protrusion 124. Accordingly, this protruding solder joint around the pins of the electrical component creates a point for a potential electrical short and for potential damage during further assembly of the overall product and during operation and repair thereof. For example, in the event that the circuit board comprises a daughter card that is to be mounted or soldered on a further system card or motherboard. Additionally, for example, the protruding solder joints produce points for potential electrical shorts with respect to jumper wires or other components that may be added to the board at a subsequent point. Further, in order to avoid future electrical shorts within the overall product, the protruding solder joints create constraints on the minimum distance between two adjacent boards in the overall assembly—thus creating constraints on such things as product size reduction. Alternatively, in the use of such traditional solder processes that create protruding solder joints, a separate substrate would be attached to the primary assembly to serve as an insulating layer to protect and insulate the protruding solder joints.
There is, therefore, a need for an electrical circuit board component solder/assembly process, and resulting circuit board product, that eliminates any solder protrusion points in a through-hole electrical component solder/assembly process.